The best college applicants are well-rounded. A student who has good grades but doesn’t participate in any extracurricular activities may not look as good to an admissions officer as one who has decent grade and also runs track, is a member of student government, and works a part-time job. Use your years in high school to build up your resume: take (and do well in) hard classes, befriend your teachers, participate in after-school activities, and express your interest to potential colleges early on.
During the summer you have plenty of time to spruce up your résumé. Find a job or internship to earn money and experience, or consider getting involved with a volunteer organization that interests you and with whom you can continue working throughout the school year. Taking classes, reading books and studying for upcoming standardized tests, and travelling are also meaningful activities for the summer.